1. Field of the Invention
The present invention relates to an electronic device, and more particularly, to a technique capable of reducing power wasted in a non-contact IC card, which is carried by a person, used for entrance and exit control, payment, and used as an electronic ticket, or in a sensor network device used for security system, distribution system, or the like, thereby improving energy use efficiency of the entire system.
2. Description of the Related Arts
As shown in FIG. 4, a conventional electronic device includes an RF coil 401 for receiving an magnetic field or a radio wave and generating power, a diode 406 and a capacitor 407 which are used for converting alternating current (AC) of the RF coil 401 into a direct current (DC), and an RF transmission circuit 404 (JP 11-055165 A).
As a point for classifying electronic devices having this configuration, there is a point as to whether or not an electronic device itself has an energy source to perform independent operation. For example, in a case of an application which is represented by an RFID chip and which targets the replacement of a bar code or a case of a railroad ticket gate system using an IC card, the electronic device itself does not have a battery and thus operates on power electromagnetically supplied to the device simultaneously with communication. Since independent operation is impossible in this application, operation area of the device is limited. The device can be used only in an area in which a corresponding reader/writer locates. Accompanying transmission of electric power limits an operating distance from a reader. In many cases, the device is made light in weight and an extremely large number of devices are used in a low-cost application. On the other hand, there is a system in which a device has a battery or an external power source to perform independent operation. Up to now, such system is used in a field of security, distribution, or the like. Such independent systems are so-called sensor network systems for sensing something and generating a notice of a result obtained by the sensing. In a case of a security system, a person or a fire is sensed. In a case of a distribution system, a temperature of a product under transportation, or the like, is sensed. As a case where the sensing is not performed, there is an application in an information transmission, such as remote control of a key in a security system or price display tag in a distribution system, which is gaining great expectation in the sensor network system.
In the conventional electronic device, an electromagnetic wave is transmitted to the sky, so a certain amount of power is necessary for both transmission and reception in radio communication. In the radio communication, only a slight part of transmitted power reaches the RF coil 401 located on a reception side. Therefore, when the received power is amplified by an RF amplifier or the like (not shown in FIG. 4), unnecessary power is consumed. Alternatively, when the power is to be received at one time, it is necessary to increase the size of the RF coil 401. In this case, there is a problem in that a total effective area of the electric device increases. In addition, when the battery or the external power source is used instead of the RF coil 401, the power is consumed even in a standby state (state in which circuits are not operated), so there is a problem in that energy use efficiency is very low.